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Higher Processing of Visual Information Lecture I
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April 2, 2007 by Mu-ming Poo
1. Overview of the Mammalian Visual System
2. Retinotopic Maps and Cortical Magnification
3. Structure Lateral Geniculate Nucleus (LGN)
4. LGN Receptive Fields
5. Organization of Mammalian Visual Cortices
6. Structure of the Primary Visual Cortex
Part I. Overview of Visual Pathway
Projections of RGCs to the central nervous system
1. Optic nerve and optic chiasm
Optic nerve consists of axons of retinal ganglion cells (RGCs).
At the optic chiasm, RGC axons from nasal hemiretina cross and enter the
contralateral hemisphere. RGC axons from temporal hemiretina enter the
ipsilateral hemisphere.
2. Mid-brain projections
RGC axons project to pretectum, which controls pupillary reflex.
RGC axons project to superior colliculus, which controls saccadic eye movement.
3. Central projections
LGN axons project to the primary visual cortex (striate cortex, V1)
on the same side
Retinotopic map
All structures in the geniculo-cortical
system have retinotopic maps.
Every point on the map corresponds to a
point on the retina.
Cortical magnification
In cortex, the area devoted to the foveal
region of the retina is enlarged relative to
the rest of the eye.
Cortex
Visual field
LGN
Visual field
Fovea: 1 cone => 1 RGC
1 RGC => 2-3 LGN cells
1 LGN cell => 1000 V1 cells
10o periphery: 2 cone => 1 RGC
2 RGCs => 1 LGN cell
1 LGN cell => 100 V1 cells
Cortex
Structure of the primate LGN
Each LGN has 6 layers labeled 1 thru 6
from ventromedial to dorsolateral.
6
5
Magnocellular LGN layers - Layers 1
& 2 - inputs from M-type ganglion cells;
4
Parvocellular LGN layers - Layers 3,4,5,6
- inputs from P-type ganglion cells;
3
2
1
Koniocellular LGN layers – Interlayers just ventral to each numbered
layer - inputs from blue/yellow
ganglion cells
50m
Layers 1,3,6 receive contralateral input and Layers 2,3,5 receive ipsilateral input.
Each LGN serves the contralateral visual field. The retinotopic maps from two
eyes are in register across the layers. (Axons from RGCs responding to the same
visual field innervate LGN cells that are aligned vertically across the layers)
Properties of P, M, and K cells
Cortical Feedback to LGN: The corticogeniculate pathway
Only <30% inputs to LGN are from
the retina. >70% are feedback
inputs, mostly from V1.
The corticogeniculate axons
terminate in all LGN layers in two
main patterns (Ichida & Casagrande,
2002)
Common functional patterns
of corticogeniculate connectivity
(Sillito et al, Science 1999):
(1) in individual M
or P LGN layers.
Feedback to LGN cells along the preferred
orientation, enhancing the buildup of orientation
selectivity within the geniculo-cortical circuit.
Feedback to LGN cells along the axis of
stimulus movement, enhancing responsiveness
in anticipation of stimulus movement.
(2) Terminate in pairs
of M, P, or K layers.
Receptive Fields of RGCs Leaving the retina
ON-OFF
Center-surround
receptive field
Color opponency
M cells
P cells
B/Y (K) cells
3. LGN receptive fields
Each LGN neuron receives input
from only a few ganglion cells.
The receptive fields of LGN
neurons are concentric with a far
stronger surround antagonism
than that in RGCs, suggesting
the LGN is not a simple relay
station.
(Hubel & Wiesel, 1962)
Brodmann’s cytoarchitectonic maps of cerebral cortex
Side view of left hemisphere
Inside view of right hemisphere from the midline
The organization of mammalian visual cortices
monkey
monkey
human
Anatomy of the primary visual cortex (V1, striate cortex)
V1 has six layers
Layer 1 is nearly aneuronal,
composed predominantly of dendritic
and axonal connections.
Layers 2/3 contain excitatory neurons
which project to extrastriate cortical
regions
Layer 4 is divided into 4A, 4B, 4Ca,
and 4Cb. Layers 4Ca and 4Cb are the
major recipients of LGN projections.
Layers 5/6 contain excitatory
projection neurons which provide
feedback to LGN.
Blobs --Cytochrome oxidase (CO)
labeling of V1 shows CO rich regions in
layer II & III termed "blobs"
Layer 3
Cortical cell types
Excitatory cells:
Spiny pyramidal & stellate cells
Inhibitory cells: >40 types
Inputs from LGN
Most LGN cells terminate in
layer 4, M in 4Ca, P mostly in
4Cb.
Collaterals (branches) of M &
P terminate in 6.
K cells and some P cells
terminate in blobs in layers 2
& 3.
Output from V1
Layers II, III & IVB - feedward to
extrastriate cortical areas
Layer V - feedback to pons
and superior colliculus
Layer VI - feedback to LGN